It is known in the prior art to use inertial motion sensors to track the position, orientation, and velocity (linear or angular) of objects in the inertial reference frame, without the need for external references. Inertial motion sensors generally include gyroscopes, accelerators, and other motion-sensing devices. Gyroscopes are well-known and used for measuring or maintaining orientation based on the principles of conservation of angular momentum. Vibrating structure gyroscopes, due to their simplicity and low cost, gained popularity since 1980s over conventional, rotating gyroscopes. The physical principle of a vibrating structure gyroscope is very intelligible: a vibrating object tends to keep vibrating in the same plane as its support is rotated. In engineering literature, this type of device is also known as a Coriolis vibratory gyro because, as the plane of oscillation is rotated, the response detected by a transducer of the device results from the Coriolis effect (as in a conventional rotating gyroscope). The Coriolis effect is an apparent deflection of moving objects from a straight path when they are viewed in a rotating frame of reference, and is caused by the Coriolis force, which is considered in the equation of motion of an object in a rotating frame of reference and depends on the velocity of the moving object, and centrifugal force. By determining the Coriolis force, a rotation of the object can be described. Both the Coriolis force and the Coriolis effect are well known in the art.
Some of vibrating gyroscopes utilize piezoelectric oscillators to capture the rotational movements of objects, (see, e.g., Ceramic Gyro™ devices by NEC-TOKIN Corporation of Japan, www.nec-tokin.com). In such conventional, bulk piezoelectric gyroscopes, used as, for example, angular velocity sensors, a piezoelectric element torsionally vibrates a rod, which causes the rod to work as a pendulum. Then, the value of the Coriolis force, which occurs when the rod is rotated, is extracted after it is converted into voltage by the piezoelectric element.
With material-micromachining becoming a rapidly developing technology in recent years, silicon (Si) based microelectromechanical systems (MEMS) and devices enriched the field of inertial sensors by offering relatively inexpensive vibrating structure gyroscopes. A general discussion of MEMS-based gyroscopes is provided, for example, by Steven Nasiri, “A critical review of MEMS gyroscopes technology and commercialization status”, ca. 2005, available at http://www.invensense.com/shared/pdf/MEMSGyroComp.pdf
To date, MEMS-based gyroscopes have been implemented in several embodiments. Some single-mass linear resonators, for example, utilize a single mass oscillating to and fro along the “sensitive” axis of the device, like a balance in a watch (see, e.g., FIG. 1, illustrating a device by HSG-IMIT, Institut für Mikro- und Informationstechnik, or Institute for Micromachining and Information Technology, of Germany, http://www.hsg-imit.de/index.php?id=41&L=1). If such linear device is rotated around an axis parallel to its sensitive axis, Coriolis forces induce a second oscillation oriented perpendicular to the direction of the (primary) oscillation of the mass.
Another variation of a linear-resonator gyroscope is based on a tuning-fork idea, for example, as implemented by the Draper Laboratory of Massachusetts, USA (www.draper.com), and described, for example, in U.S. Pat. Nos. 5,767,405 and 7,043,985, each of which is incorporated herein in its entirety by reference. An example of a basic tuning fork MEMS gyroscope, shown in FIG. 2, includes a pair of masses driven to oscillate with equal amplitudes but in opposite directions. Rotation of the gyroscope about an in-plane axis of sensitivity lifts the moving masses, which is detected with capacitive electrodes positioned under the masses.
Analog Devices Inc. of Norwood, Mass. offers a number of integrated angular-rate sensing gyroscopes (see, for example, Geen et al., “New iMEMS® Angular-Rate-Sensing Gyroscope”, Analog Dialog, Vol. 37, March 2003, available at http://www.analog.com/library/analogDialogue/archives/37-03/gyro.html, which is hereby incorporated herein by reference in its entirety). Some exemplary MEMS gyroscopes are described in U.S. Pat. Nos. 6,877,374, 7,089,792, 7,032,451, 7,204,144, 7,357,025, and 7,216,539, each of which is hereby incorporated by reference in its entirety. In such gyroscopes, capacitive silicon sensing elements are interdigitated with stationary silicon beams attached to a substrate that are used to measure a Coriolis-induced displacement of a resonating mass.
Another family of MEMS-based inertial sensors known in the art includes vibrating-wheel gyro structures, schematically illustrated in FIG. 3. These structures generally have a wheel driven to vibrate about its axis of symmetry, where rotation about either in-plane axis results in the wheel's tilting, a change that can be detected with capacitive electrodes under the wheel. Yet another emerging MEMS-implementation is a ring gyroscope, where a planar resonant Si-based ring structure is driven to resonance and the position of its nodal points indicate the rotation angle. An example of a ring gyroscope shown in FIG. 4 was developed at the University of Michigan (see, e.g., F. Ayazi and K. Najafi, “Design and Fabrication of A High-Performance Polysilicon Vibrating Ring Gyroscope”, Eleventh IEEE/ASME International Workshop on Micro Electro Mechanical Systems, Heidelberg, Germany, Jan. 25-29, 1998; F. Ayazi and K. Najafi, “High aspect-ratio combined poly and single-crystal silicon (HARPSS) MEMS technology”, J. of Microelectromechanical Systems, v. 9, pp. 288-294, 2000; F. Ayazi and K. Najafi, “A HARPSS Polysilicon Vibrating Ring Gyroscope”, Journal of Microelectromechanical Systems, Vol. 10, No. 2, June 2001). In such ring gyroscopes, a poly-Si ring resonator is driven by drive and control electrodes to vibrate about its axis of symmetry, and rotation of the chip that carries the gyroscope results in a ring displacement that can be detected by capacitance-sensing elements through the change in geometry of capacitive air-gaps.
As would be appreciated by one skilled in the art, currently employed micromachining processes are not particularly compatible with widely used standard CMOS processes such as reactive-ion etch (RIE) or electron-beam milling. Such incompatibility lengthens and complicates fabrication cycles and increases the cost of the resulting MEMS-based devices. In addition, current MEMS-based solutions for inertial sensors quite often employ capacitive driving and sensing structures, such as drive and control electrodes of FIG. 4. The capacitive nature of operating a conventional inertial sensor imposes, among other requirements, a need for an air-gap between an oscillating mass (the ring in FIG. 4) and an electrode. The air-gaps of MEMS structures are clearly susceptible to contamination with microparticles (during both the manufacturing process and operation) that may permanently incapacitate inertial sensors devices. Moreover, an electrostatically driven mass or ring in such devices is susceptible to anomalies in charge distribution across a set of driving and sensing electrodes. In vibrating gyroscopes, the non-uniform charge distribution can contribute to an offset drift and reduce the accuracy and precision of these devices or even nullify their performance.